The present invention relates to turbomachine blades. It relates to the turbomachine blades or turbomachine modules, such as compressor or turbine modules, that are cast, and in particular it relates to the fattening of the connection zone between the airfoil and its heel or the airfoil and the associated platform.
The blades that are cast in nickel-based or cobalt-based superalloys are manufactured according to the technique called cire perdue. These blades have columnar or monocrystalline metallurgical structures obtained by a method of directional solidification. This method is awkward to control, particularly for hollow and greatly three-dimensional parts.
The manufacture of such blades involves the production of a model made of wax or another equivalent material, which includes an inner part forming a casting core and depicting the cavities of the blade. To form the model, an injection mold for wax is used in which the core is placed and the wax is injected into it. The wax model is then dipped several times in slurries made of a suspension of ceramic particles in order to produce a shell mold. The wax is removed and the shell mold is baked. The blade is obtained by pouring a molten metal which occupies the empty spaces between the inner wall of the shell mold and the core. Thanks to a nucleus, or an appropriate selector, and controlled cooling, the metal solidifies into a desired crystalline structure. Depending on the nature of the alloy and the expected properties of the part resulting from the cast, it may be a columnar-structure directional solidification, a monocrystalline-structure directional solidification or equiaxis (EX) solidification. After the solidification of the alloy, the shell and the core are knocked out, and the desired blade emerges.
The solidification is a time during which the metal sustains considerable thermal stresses; these stresses often cause the metal to recrystallize. Specifically, on raw cast monocrystalline solid blades, considerable zones of well-defined recrystallization are found. For example, when the blade has a heel, there are recrystallized zones on the airfoil, under the heel, approximately 10 mm under the airfoil-heel connection, as is shown in FIG. 1. The cause of the recrystallization on these blades is the excessive stresses imposed on the metal during solidification.
To attempt to remedy this problem and remove the defect of recrystallization, several tests have been run that have not provided concrete solutions. The size of the casting supplies has been reduced or else the wall of the shell mold has been lightened. Another method tested to remedy this problem has been to add various types of extra thicknesses or swellings locally over the whole periphery of the airfoil immediately beneath the heel. Such a fattening is obtained by changing the wax model from which the model is made. Examples of changes to the roots are shown in FIGS. 2a, 2b and 2c. 
The root, or extra thickness, is defined by a height and a thickness on the pressure side and the suction side. The connection zone extends over the whole periphery of the airfoil. Tests have made it possible to find the influence of the geometry of the root on the recrystallization. FIG. 2a shows a root with a thickness a of 2 mm and a height h of 5 mm. FIG. 2b shows a root with a thickness a of 2 mm and a height 2h of 10 mm. FIG. 2c shows a root with a thickness a′ of 3 mm and a height 2h of 10 mm. It was possible to eliminate the recrystallization phenomenon only with a root of a relatively large size making it easier to cast the molten metal between the airfoil and the heel.
But, because of this size, the root is not satisfactory from an aerodynamic point of view: on the one hand, it creates a nominal tangential step in the stream, generated by the truncation of the outer stream radius, and, on the other hand, its presence over the whole profile considerably disrupts the aerodynamic performance of the turbomachine.
In addition, such a root induces no small increase in weight and an abrupt increase in the law of the section. The main consequences are, on the one hand, an increase in the centrifugal stresses on the airfoil and therefore a sharp reduction in service lives, notably in creep, and, on the other hand, an incorrect positioning of the center of gravity in a section at the heel, meaning an increase in the local stresses of the airfoil under the heel or on the disk, bringing a reduction in the service life and in the overspeed margin.